第 5 章 包管理系统：工具和基本原则

Debian 软件包格式被这样设计，在具有经典命令如 ar、tar 和 xz 或者有时候是 gzip 或 bzip2的 Unix 系统上，它的内容都可以被解压。这是个看起来琐碎的属性对移植和灾难恢复是非常重要的。

Imagine, for example, that you mistakenly deleted the dpkg program, and that you could thus no longer install Debian packages. dpkg being a Debian package itself, it would seem your system would be done for... Fortunately, you know the format of a package and can therefore download the .deb file of the dpkg package and install it manually (see sidebar 工具 dpkg、APT 和 ar). If by some misfortune one or more of the programs ar, tar or gzip/xz/bzip2 have disappeared, you will only need to copy the missing program from another system (since each of these operates in a completely autonomous manner, without dependencies, a simple copy will suffice). If your system suffered some even more outrageous misfortune, and even these don't work (maybe the deepest system libraries are missing?), you should try the static version of busybox (provided in the busybox-static package), which is even more self-contained, and provides subcommands such as busybox ar, busybox tar and busybox xz.

In case of a misfortune, you better also have a backup of your system (see 第 9.10 节 “备份”).

# ar x dpkg_1.21.22_amd64.deb
# tar -C / -p -xJf data.tar.xz

来看看 .deb 文件的内容：

$ ar t dpkg_1.21.22_amd64.deb
debian-binary
control.tar.gz
data.tar.xz
$ ar x dpkg_1.21.22_amd64.deb
$ ls
control.tar.gz  data.tar.xz  debian-binary  dpkg_1.21.22_amd64.deb
$ tar tJf data.tar.xz | head -n 16
./
./etc/
./etc/alternatives/
./etc/alternatives/README
./etc/cron.daily/
./etc/cron.daily/dpkg
./etc/dpkg/
./etc/dpkg/dpkg.cfg
./etc/dpkg/dpkg.cfg.d/
./etc/logrotate.d/
./etc/logrotate.d/alternatives
./etc/logrotate.d/dpkg
./lib/
./lib/systemd/
./lib/systemd/system/
./lib/systemd/system/dpkg-db-backup.service
$ tar tJf control.tar.xz
./
./conffiles
./control
./md5sums
./postinst
./postrm
./prerm
$ cat debian-binary
2.0

如你所见， Debian 包的 ar 存档格式由三个文件组成：